基础版

青蛙一次可以跳1级或者2级台阶，问跳到N级有多少种跳法

class Solution:
    def jumpFloor(self,n):
        if n < 1:
            return 0
        if n == 1 or n == 2:
            return n
        a = 1
        b = 2
        tmp = 1
        for i in range(2,n):
            tmp = a+b
            a = b 
            b = tmp           
        return tmp

Solution().jumpFloor(5)

升级版

青蛙一次可以跳1级或者n级台阶，问跳到N级有多少种跳法

class Solution:
    def jumpFloor(self,num):
        # f(n) = f(n-1) + f(n-2) + ... + f(1)
        # f(n-1) = f(n-2) + f(n-3) + ... + f(1)
        # f(n) = 2f(n-1)*n
        # return pow(2,num - 1)
        if num == 1:
            return 1
        ret = 1
        a = 1
        for i in range(2,num + 1):
            ret = 2 *a
            a = ret
        return ret
Solution().jumpFloor(4)